The invention described herein is based on a Joint Research Agreement between Bridgestone Corporation and Kuraray Co., Ltd.
1. Field of the Invention
The present invention relates to an innerliner for pneumatic tires which is superior both in gas barrier property and in flexing resistance. Furthermore, the present invention relates also to a tire using the above-mentioned innerliner and to a technology of improving the internal pressure retainability before use and after driving without increasing the weight of the tire.
2. Description of the Related Art
For the purpose of retaining the internal pressure of tires, a butyl rubber, a halogenated butyl rubber and the like are used conventionally as a major raw material of an innerliner as an air barrier layer. However, innerliners made of rubber compositions containing those rubbers are required to have a thickness of about 1 mm because of poor gas barrier properties of the rubber compositions. An innerliner, therefore, accounts for about 5% by weight of a tire. This inhibits the improvement of automobile fuel efficiency through weight reduction of tires.
On the other hand, ethylene-vinyl alcohol copolymers, which henceforth may be abbreviated as EVOH, are known to be superior in gas barrier property. EVOH has an air transmission rate of not more than 1/100 that of a butyl-based innerliner rubber composition. It, therefore, can greatly improve the internal pressure retainability even if it has a thickness of 50 μm or less. In addition, it can reduce the weight of a tire. There are many resins having an air transmission rate smaller than that of a butyl rubber. However, when a resin has an air transmission rate of about 1/10 that of butyl-based innerliners, only a small effect of improving the internal pressure retainability is achieved unless the thickness thereof is over 100 μm. When the thickness of the resin is over 100 μm, a small effect of reducing the tire weight is achieved and a rupture or crack is generated in an innerliner from its deformation occurring during the flexure of a tire. It, therefore, becomes difficult to retain the barrier property. On the other hand, because EVOH can be used even in a thickness of 50 μm or less, use of such thin EVOH will make a rupture or crack hard to be generated even if it is flexurally deformed during rotation of a tire. The use of EVOH for a tire innerliner, therefore, is effective for improving the air permeability of a pneumatic tire. As a pneumatic tire having a tire innerliner made of EVOH, known is a technology disclosed in JP-A-6-40207.
Use of a normal EVOH as an innerliner results in a great effect of improving the internal pressure retainability. However, the normal EVOH has an elastic modulus extremely higher than those of rubbers usually used for tires. Therefore, a rupture or crack may be generated in the innerliner due to its deformation caused by its flexure. For such reasons, when an innerliner made of EVOH is used, although the internal pressure retainability of a tire before use is greatly improved, the internal pressure retainability of a tire after use, which was applied with flexural deformation during the rotation of the tire, may be worsened than that before use. In order to solve this problem, disclosed is an innerliner for an inner surface of a tire which comprises a resin composition comprising from 60-99% by weight of an ethylene-vinyl alcohol copolymer having an ethylene content of from 20-70 mol % and a degree of saponification of 85% or more and from 1-40% by weight of a hydrophobic plasticizer (JP-A-2002-52904).
In late years, however, innerliners having a higher flexing resistance than conventional tire innerliners made of EVOH while maintaining the gas barrier property are coming to be required. Hence, further improvement in technology has been expected.